Heat seal films are known to be used in the packaging industry to seal many types of packaging. They are often used to seal plastic trays, or thermo-formed blister packaging. The heat seal film generally comprises a first side having a printed surface facing outwardly from the packaging and a second side having a sealing surface facing inwardly to the packaging.
The sealing surface is generally provided by a sealing layer which when exposed to heat, time and pressure seals to an abutting surface of a tray or blister pack. Alternatively, it is known to ultrasonically weld a film to a tray or blister pack.
Because it is desirable to seal packages, the known films often comprise of a number of discrete layers, with each layer performing a specific function within the film. For example, a first a layer of a film adjacent to a tray will generally be a sealing layer, and one or more other layers may provide a printed layer and/or a protection layer to protect the film during heat sealing.
It is required that some heat seal films appear to be printed/decorated on both sides so that the film has a pleasing appearance. However, in light of the fact that the inner facing surface of the film must provide a heat seal layer, any print or ink applied to the package facing surface of the film has been found to interfere with the sealing process and prevent the film sealing properly.
Until now, this problem has been solved in part by providing a colourless or clear transparent sealing layer which is laminated to a printed layer.
Furthermore, in the event that both sides of the film should appear to be printed so that the inside and outside of the packaging should appear to be printed, this currently requires the printed layer to be printed on both sides and this has required either:                a) printing the film in two passes through a printing press, each pass to print an opposing side of the film, and then laminating a transparent sealing layer to the printed film in a separate operation; or        b) printing both sides of the film through a printing press by printing one side of the film, turning it through 180 degrees and feeding it back through the printing press to print an opposing side of the film, and then laminating a transparent sealing layer to the printed film in a separate operation.        
Use of the second method mentioned above has been limited, because in the case of many films there are technical difficulties associated with turning the film 180 degrees without causing stretching, creasing or scratching of the film.
In light of these problems, various proposals have been made, but there remains a need for an improved printing press and methods which address one or more of the problems presented by prior art arrangements.
Remarkably, the invention provides a new printing press for printing heat seal films wherein the printing press is capable of printing both sides of a print layer in a single pass through the press and laminating the print layer to a sealing layer to provide a heat seal film printed on both sides. Advantageously, designs printed on first and second sides of the film can be printed in register to each other.